import math

def great_circle(float lon1,float lat1,float lon2,float lat2):
    cdef float radius = 3956.0
    cdef float pi = 3.14159265
    cdef float x = pi/180.0
    cdef float a,b,theta,c

    a = (90.0-lat1)*(x)
    b = (90.0-lat2)*(x)
    theta = (lon2-lon1)*(x)
    c = math.acos((math.cos(a)*math.cos(b)) + (math.sin(a)*math.sin(b)*math.cos(theta)))
    return radius*c
